The following abbreviations are herewith defined, at least some of which are referred to within the ensuing description of the prior art and the present invention.    CQI Channel Quality Information    HS-DSCH High-Speed Downlink Shared Channel    HS-SCCH High-Speed Shared Control Channel    LTE Long Term Evolution    Mbps Megabits Per Second    MCS Modulation and Coding Scheme    MIMO-SCCH Multiple Input Multiple Output Shared Control Channel    MMSE Minimum Mean-Square Estimation    OFDM Orthogonal Frequency Division Multiplexing    PARC Per-Antenna Rate Control    SNR Signal-Noise-Ratio    S-PARC Selective Per-Antenna Rate Control    SIC Successive Interference Cancellation    TTI Total Timeslot Interval    WCDMA Wideband CDMA
In the multi-antenna concept proposed for the HSDPA mode of a WCDMA cellular system, the selection of a subset of antennas by the base station from which to transmit data streams to mobile terminals is considered an extension of fast link adaptation for single-antenna systems. This multi-antenna approach leads to a transmission antenna mode that is closely matched to the existing propagation conditions between the base station and mobile terminals. In one case, the base station can utilize PARC which is a multi-antenna approach that provides high downlink data rates to mobile terminals (see, S. J. Grant et al. “Per-Antenna-Rate-Control (PARC) in Frequency Selective Fading with SIC-GRAKE Receiver” Los Angeles, September 2004). In this approach, the base station uses separate transmission rates for the data streams which are mapped to all of the transmit antennas. In another case, the base station can utilize S-PARC which is a multi-antenna approach that provides even higher downlink data rates to the mobile terminals (see, S. J. Grant et al. “System-Level Performance Gains of Selective Per-Antenna-Rate-Control (S-PARC)” published in the IEEE Spring Vehicular Technology Conference, May 2005). In this approach, the base station selectively adapts both the transmit antenna(s) and the transmission data rates which are to be used to transmit the data streams to the mobile terminals.
As part of the fast link adaptation process, the mobile terminals measure the CQI and transmit the CQI measurements on the uplink to the base station so that the base station can use the CQI measurements to select the transmission data rate assignment and if needed the transmit antenna(s) assignment. Since the base station transmits signals concurrently from different transmit antennas these signals interfere with each other which means the estimated CQI is going to change for each combination of transmit antennas. Thus, the providing of CQI estimates for each transmit antenna under each combination of transmit antenna(s) is going to utilize a large amount of the available uplink resources when compared to single-antenna transmission. Moreover, the type of receivers employed in the mobile terminals may increase the number of CQI values that may be fed back on the uplink to the base station. For example, the mobile terminal may have a SIC receiver which places an ordering on the transmit antenna(s) so that there will be a separate CQI value for each permutation (rather than combination) of the transmit antenna(s).
One approach that can be used to reduce the complexity of the CQI feedback from a mobile terminal that uses a SIC receiver and implements S-PARC was described in co-assigned U.S. patent application Ser. No. 10/841,911 filed on May 7, 2004 and entitled “Reduced CQI Feedback for MIMO HSDPA” (the contents of which are hereby incorporated by reference herein). In this approach, the base station that is considering transmission with one antenna will select the transmit antenna that provides the best rate and use that for a single-antenna transmission. In considering transmission with two antennas, the base station constrains the two-antenna subset to contain the best antenna previously found for the single-antenna transmission and the transmit antenna with the next-best rate. This scheme is repeated for the three-antenna subset, the four-antenna subset etc. . . . , and in general is called the “subset property” when related to the transmit antenna selection. Thus, an order is given to the transmit antennas so that the first antenna has the greatest transmit rate while the last antenna has the lowest transmit rate. This ordering of antennas from lowest to greatest rates is the order that the SIC-receiver processes the received signals. As a result, a large number of antenna orderings can be avoided and the feedback on the uplink can be reduced by constraining the antenna order and antenna subset selection by using a CQI report with this subset property.
Moreover, if the base station acquires the CQI report based on the subset property then it can use this scheme to help determine the set of antennas which are to be used for transmission. Thereafter, the base station signals to the mobile terminal the number of antennas used for transmission and the transport formats for the transmitted streams. Since, both the base station and the mobile terminal know the ordering contained as part of the CQI report, the actual transmit antennas used for the transmission could be determined by the mobile terminal. This notion of antenna ordering can also be used with mobile terminals/receivers that do not require an ordered set of antennas for detection. In this situation, the antenna ordering is used solely for providing an efficient CQI reporting technique and allows the base station to select the number of transmit antennas and the corresponding rates for subsequent transmissions.
In WCDMA, release 5, the base station can use four HS-SCCHs and different channelization codes to transmit the transport information to as many as four different mobile terminals during a single TTI. While, the co-pending U.S. patent application Ser. No. 11/275,388 introduced a MIMO shared control channel (MIMO-SCCH) which the base station could use to signal the transport information to a mobile terminal that is capable of receiving a multi-stream transmission. In the HSDPA mode, the base station reliably transmits the HS-SCCHs and MIMO-SCCH by using a spreading code of 128 to ensure the strength of the despread signal. In addition, the base station can place the HS-SCCHs and MIMO-SCCH on the best transmit antenna to help improve the SNR of these control channels. However, in a LTE mode which is based on OFDM there is no such spreading code which can be used to boost the signal level of one antenna relative to the other transmit antennas (note: WiMAX and 4G systems are other types of OFDM systems). Thus, to help ensure the reliable transmission of the shared control channel and information thereon from one antenna to the mobile terminal(s), the base station could use a couple of different approaches as follows:                For those OFDM symbols corresponding to the shared control channel, allow no other antenna to transmit at the same symbol positions. This, of course, denies use of the same OFDM tiles for the transmission of data.        The information on the shared control channel can be encoded using a stronger code rate to overcome interference from the other transmit antennas. However, this may not be the most effective approach since the control channel already has strong coding and this coding would be used to overcome interference that has been self-generated.        Use interference cancellation techniques at the mobile terminal to cancel the interfering signals from the other transmit antennas. However, to cancel the self-interference generated by multiple transmit antennas, the mobile terminal has to know which antennas are used for data transmission and for the control channel so the appropriate quantities (e.g. impairment covariance matrix) can be constructed at the receiver.        Transmit the control channel on the “best” antenna. In the co-pending U.S. patent application Ser. No. 11/275,388, the best antenna is the one with the highest SNR as reported within the CQI measurements. However, when there are multiple CQI measurements reported, for example, over different sub-bands in an OFDM system, then this notion of best antenna is ambiguous. Also, in co-pending U.S. patent application Ser. No. 11/275,388, the SNR associated with an antenna may be related to the order the antennas are detected because it is generally desired to have control channels detected prior to the data channels.        
Thus, while it is clear that using the best transmit antenna to advantageously transmit the control channel should be performed, the decision about which antenna is best and the signaling required to support this transmission needs further consideration. These particular needs and other needs are satisfied by the base station and method of the present invention.